Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. OCT is a method of interferometry that uses light containing a range of optical frequencies to determine the scattering profile of a sample. Optical coherence tomography (OCT) as a tool for evaluating biological materials was first disclosed in the early 1990's (see U.S. Pat. No. 5,321,501 for fundus imaging.). Since that time, a number of manufacturers have released products based on this technology. For example, the assignee herein markets a device called the StratusOCT. This device is used for diagnostic imaging and provides direct cross sectional images of the retina for objective measurement and subjective clinical evaluation in the detection of glaucoma and retinal diseases. The device can generate images of macular thickness, the retinal nerve fiber layer, the optic disc, the cornea, and other parts of the eye. This device is based on a version of OCT known as time domain OCT.
In recent years, it has been demonstrated that frequency domain OCT has significant advantages in speed and signal to noise ratio as compared to time domain OCT (Leitgeb, R. A., et al., Optics Express 11:889-894; de Boer, J. F. et al., Optics Letters 28: 2067-2069; Choma, M. A., and M. V. Sarunic, Optics Express 11: 2183-2189).
In frequency domain OCT, a light source capable of emitting a range of optical frequencies excites an interferometer, the interferometer combines the light returned from a sample with a reference beam of light from the same source, and the intensity of the combined light is recorded as a function of optical frequency to form an interference spectrum. A Fourier transform of the interference spectrum provides the reflectance distribution along the depth within the sample.
Several methods of Frequency domain OCT have been described in the literature. In spectral-domain OCT (SD-OCT), also sometimes called “Spectral Radar” (Optics letters, Vol. 21, No. 14 (1996) 1087-1089), a grating or prism or other means is used to disperse the output of the interferometer into its optical frequency components. The intensities of these separated components are measured using an array of optical detectors, each detector receiving an optical frequency or a fractional range of optical frequencies. The set of measurements from these optical detectors forms an interference spectrum (Smith, L. M. and C. C. Dobson, Applied Optics 28: 3339-3342), wherein the distance to a scatterer is determined by the wavelength dependent fringe spacing within the power spectrum. SD-OCT has enabled the determination of distance and scattering intensity of multiple scatters lying along the illumination axis by analyzing a single the exposure of an array of optical detectors so that no scanning in depth is necessary. Typically the light source emits a broad range of optical frequencies simultaneously. Alternatively, in swept-source OCT, the interference spectrum is recorded by using a source with adjustable optical frequency, with the optical frequency of the source swept through a range of optical frequencies, and recording the interfered light intensity as a function of time during the sweep (U.S. Pat. No. 5,321,501).
The commercial OCT systems typically include some form of scanning mirror configuration to scan the light beam across the eye in a plane perpendicular to the propagation axis of the beam. The most common interferometer configuration for OCT is the Michelson interferometer [FIG. 1a of U.S. Pat. No. 5,321,501]. Michelson interferometers return some reference arm light to the source, causing a conflict between the desire to set the reference level for best performance of the detector, and to set the reference level low enough to be below the back-reflection tolerance. Some alternative interferometer topologies allow the reference path to be completely in fiber, allowing simple construction. If the reference path is completely in fiber then the sample path length can be varied instead (U.S. Pat. No. 5,321,501).
Non-reciprocal optical elements in the source arm [U.S. Pat. No. 6,657,727 issued to Izatt, et al.] have been used to divert the reflected light that would otherwise return to the source to a detector. While this protects the source and increases its longevity, non-reciprocal optical elements in the source arm add significant costs to the interferometer manufacture.
Interferometers with topology different from the common Michelson topology have been proposed for OCT (U.S. Pat. No. 5,321,501 FIG. 10, U.S. Pat. No. 6,201,608 issued to Mandella, et al., and U.S. Pat. No. 6,992,776 issued to Feldchtein, et al.). Some of these designs route the reference light without retro-reflecting or otherwise reversing the reference light back toward the source. In such interferometer designs some light returned from the sample can reach the source, but this is less of a concern because in many applications only a small fraction (10−4 to 10−10) of the incident light is scattered from the sample and returned to the interferometer.
There has been a continuing effort in the industry to improve the existing OCT systems. For example, when measuring living tissue such as an eye, movement during the measurement period can cause a wide variety of difficulties. Efforts have been made to increase the speed of data collection to reduce the effects of motion of the subject. In addition, various approaches have been suggested to measure sample motion and then compensate for that motion.